The present invention relates to an optical scanning device comprising an optical objective lens. More specifically, but not exclusively, the invention relates to an optical scanning device capable of writing and reading data from two different types of optical record carriers, such as compact discs (CDs) and digital versatile discs (DVDs), with laser radiation of a different wavelength, respectively, using the same optical objective lens system.
It is desirable for an optical scanning device to be capable of recording and reproducing optical record carriers of different formats. CDs are available, inter alia, as CD-As (CD-audio), CD-ROMs (CD-read only memory) and CD-Rs (CD-recordable). CDs are designed to be scanned with a wavelength of about 780 nm and a numerical aperture (NA) of 0.45, DVDs are designed to be scanned at a wavelength in the region of 660 nm. For reading DVDs an NA of 0.6 is generally used, whereas for writing DVDs an NA of 0.65 is generally required.
DVDs and CDs differ in the thickness of their transparent substrates, providing different information layer depths. The information layer depth for DVD is about 0.6 mm, whereas the depth for CD is about 1.2 mm. Thus if CDs are read with an optical scanning device with an objective lens optimised for DVD, a large amount of spherical aberration results at the information layer. It is possible to compensate for this effect when using a single objective lens system and a laser beam of 660 nm wavelength for reading both CD-ROMs and DVDs by reducing the numerical aperture (NA) for reading CD-ROMs from about 0.45 to 0.38, causing the spherical aberration to be within the limits for a proper reconstruction of the information stored on the CD-ROM. However, if in addition CD-Rs are to be scanned by the optical scanning device, a 780 nm laser beam has to be applied, because the CD-R is designed for writing and reading specifically at that wavelength. For CD-R organic dye is used as a recording film, of which the reflection characteristics change significantly with wavelength. It is difficult to achieve sufficient modulation for reflected radiation of 660 nm to reconstruct the information stored on a CD-R. Using a 780 nm laser beam and an NA of 0.45 for reading CDs with the same objective lens as used for DVD causes a large spherical aberration. Therefore the spherical aberration has to be compensated in some way in order to achieve an optical storage device capable of reading and/or recording CD-R, CD-ROM and DVD using laser radiation of 660 nm and 780 nm with a single optical objective lens.
Systems capable of reading DVD and CD by using laser radiation of different wavelength with the same objective lens are known in the art. WO 99/57720 describes such a system, which uses a moulded plastic lens having either two refractive aspheric surfaces or one aspherical surface and one refractive spherical surface including a diffractive element. The lens is capable of correcting for spherical aberration caused by the difference in thickness for the two disc formats as well as for chromatic aberration. However, the major drawbacks of applying plastic is its large temperature sensitivity and its large dispersion or, equivalently, low Abbe number. The refractive index change for plastic as a function of temperature is about ten times larger than for that glass. Large dispersion means large sensitivity to wavelength variation. In an optical storage device there are several sources of wavelength change: A spread in laser wavelength for mass produced lasers (xcex94xcex≈xc2x15 to xc2x110 nm), the bandwidth of the lasers (xcex94xcex≈2 nm) and wavelength dependence on output power (xcex94xcex/dP≈0.2 nm/mW).
Especially the last issue makes the application of refractive plastic lenses in rewritable systems very hard, if not impossible. During write operation, the laser switches from low power to high power in a few microseconds, causing defocus due to the wavelength change and dispersion. The mechanical bandwidth of a focus actuator is too low to compensate for this defocus, and, consequently, the data is not written properly. To avoid this problem glasses with high Abbe numbers are usually selected, which are in general expensive ones.
For conventional glass lenses, manufactured in a replication process the spread in laser wavelength is generally not a problem, since the lens actuator can generally compensate for the stationary defocus it introduces. In diffractive or hybrid lenses, on the other hand, this spread is a potential problem, because it not only leads to defocus, but also to spherochromatism and thus limits the performance of the lens.
WO 99/57720 also does not describe a system capable of writing data to optical record carriers of DVD format due to the numerical aperture of the objective lens: to properly write data at sufficient density to realise 4.7 Gbyte per disc, the NA has generally to be increased from 0.60 to 0.65. This increase in numerical aperture makes the design of an objective lens for a system capable of reading and writing data to DVD and scanning CD, which is corrected for spherical aberration, defocus due to fast wavelength variation and spherochromatism, much more difficult. High-NA lenses made of plastic material can only be realised with small radii of curvature and more than one aspherical surface, which makes the lens inherently more difficult and more expensive to manufacture.
In U.S. Pat. No. 5,349,471 a diffractive/refractive hybrid lens for use in an optical data storage system is described, which is made of a high dispersive glass with thus a low Abbe number. The resulting increased longitudinal chromatic aberration is compensated by the diffractive lens element. The described objective lens is for example able to correct for wavelength variations over a 20 nm wavelength range and up to 10 nm bandwidth of a laser diode. However, the lens is not capable of reading and writing data from optical record carriers of different thicknesses.
In U.S. Pat. No. 5,838,496 a diffractive multi-focal objective lens is described. A first focal point is formed by radiation of a lower diffractive order, whereas radiation of a higher diffractive order converges at a second focal point on the optical axis nearer to the lens than the first focal point. In addition the lens compensates for chromatic aberration and is thus capable of writing and reading data from optical discs with different thicknesses of the transparent substrate. However, the system is not designed for the use of radiation of two different wavelengths as it is needed to be able to scan data on DVD, CD-A, CD-R and CD-ROM formats with the same objective lens in an optical scanning device.
EP 936604 describes an optical pickup device suitable for reading and writing discs of DVD, CD-R and CD-ROM format with laser radiation of two different wavelengths. For this purpose an optical element having a first diffractive element in its central region and a second diffractive element with a different grating structure in its peripheral region is used in addition to an objective lens. A stepped grating profile has been chosen as a diffractive element. The central region of the optical element allows transmission of laser radiation of a first wavelength without any change and increases the diameter of laser radiation of a second wavelength. The peripheral region again allows transmission of laser radiation of the first wavelength without any change and simultaneously prevents the radiation of the second wavelength from contributing to the spot formation. In this way the NA for use of radiation of the second wavelength is reduced to a desired value.
It is a disadvantage of the system described that a stepped grating profile has been chosen which is usually manufactured in a photolithographic process. It is difficult to manufacture stepped grating profiles of small dimension in a replication process. It is a further disadvantage of the system described, that the additional optical element comprises two regions with different diffractive properties, which makes manufacturing of such a device more complicated.
It is an object of the invention to provide improved optical scanning devices for scanning optical record carriers.
According to one aspect of the present invention there is provided an optical scanning device for scanning first and second optical record carriers with a first and a second information layer depth by radiation of a first wavelength xcex1, and a second wavelength xcex2, respectively, wherein, preferably, 620 nm less than xcex1 less than 700 nm and 740 nm less than xcex2 less than 820 nm, said device being capable of reading and writing optical record carriers of said first format, the device including an objective lens (10) with at least one refractive element and at least one diffractive element (11) and with a numerical aperture NA of NA greater than 0.60 for said first wavelength xcex1, said diffractive element including a pattern of generally sawtooth-like elements; wherein the diffractive and refractive properties of the objective lens are selected: such that radiation of said first wavelength is transmitted and focused on optical record carriers of said first thickness; such that radiation of said second wavelength is transmitted and focused on optical record carriers of said second thickness; and such that a variation in wavelength xcex1 of 5 nm results in spherochromatism below 0.03xcex1.
In this way an optical scanning device is achieved capable of scanning data on record carriers of a first and second thickness with a first and second wavelength respectively. The objective lens is designed to change a parallel beam of wavelength xcex1 into a converging beam of NA greater than 0.60, which ensures that the system is capable of writing data with a high density.
The present invention is capable of providing a scanning device with the following features:
the system is capable of writing data to record carriers of a first format because it is corrected for chromatic aberration resulting from fast wavelength variations during write operations;
it has a diffractive element with a generally sawtooth-like pattern, such that it may for example be manufactured in a single-step replication process;
the system has high efficiency for scanning first optical record carriers (e.g. DVDs) and acceptable efficiency for scanning second optical record carriers (e.g. CDs); and
the lens provides limited spherochromatism and the system is thus able to cope with wavelength variations. Preferably, the diffractive and refractive properties are further selected such that the amount of defocus due to wavelength variations in xcex1 of xcex94xcex=2 nm is below 0.03 xcex1.
This ensures that the amount of defocus due to fast wavelength variations as a result of switching the power of the laser during write operation is limited as the defocus arising from these fast wavelength variation cannot be compensated by a focus actuator.
Preferably, the sawtooth-like pattern elements have a width of at least 10 xcexcm.
This ensures an easy manufacturability in a single-step replication process. The grating structure may be incorporated into the mould by a finite-sized turning tool.
According to another aspect of the present invention there is provided an optical scanning device for scanning optical record carriers of a first and second format by radiation of a first and second wavelength xcex1 and xcex2, respectively, the device including an optical objective lens with at least one refractive and one diffractive element, wherein the refractive and diffractive properties are selected such that diffractive orders m1 and m2 of wavelength xcex1 and xcex2 are used to scan optical record carriers of said first and second format respectively with: (m1, m2) being one of the following combinations: (m1, m2)=(xe2x88x922, xe2x88x922), (xe2x88x923, xe2x88x922), (xe2x88x923, xe2x88x923) or (xe2x88x924, xe2x88x923).
In this way a highly suitable device can be achieved capable of writing and reading optical record carriers of a first and second thickness by radiation of a first and second wavelength xcex1 and xcex2, respectively, whilst the (xe2x88x921, xe2x88x921) prior art solution is excluded.